Why brownfield resignalling projects always require a transition state

Author: Nick Terry
Day: Introduction Day
Session: Project Implementation

Why Brownfield Re-signalling Projects always require a Transition State
This paper concerns the process of changing the train control system on an operating railway (a
brownfield resignalling project) whilst maintaining resilience during the difficult period of change
(the transition period).
The overall objective of a resignalling project is to change from the existing operating state to a new
operating state in order to realise safety and/or business benefits. Making the whole of the change
in one step overnight is not feasible on an operating railway. Whilst it may be practicable to replace
the whole of the signalling technology over one weekend, it is not feasible to make major changes to
multiple staff working practices, safe operating procedures and rail timetables over that same
weekend. The risk of failure would be too high, both in consequence and probability, and a fallback
state is not available if only part of the transformation is successful. This leads to a step-wise
approach to the change, passing through one or more ‘transition’ states.
Train control solutions require signalling and telecommunication products, application engineering
of those products (adaptation), and people to interact with the technology. In this context, the key
people include train drivers, network controllers, maintainers, and operations managers. Any
resignalling solution needs to consider all three layers. The challenges with and within each layer are
different. The paper considers the effects on each of these three layers.
Conventional resignalling projects have always traditionally included a transition state. This would
typically involve new signals being erected in advance of the changeover weekend and covered with
a hood and a white cross. This is considered a change of infrastructure to the train drivers, so they
would be advised of the presence of non-commissioned signals via special operating notices.
As signalling technology has moved onto the train, the changes required when replacing signalling
technology have become more complicated. The number of players interacting with the signalling
system has increased, and the commercial arrangements between those players has become more
complex. The result of this is that the ability to change the whole system over one weekend has
There are only two solutions to this challenge: one solution is to introduce a significant closure of
the whole system (typically three months or more); the alternative is to break the change into a
number of steps, each of which is manageable over a weekend. This introduces temporary
operating states (Transition Stages) between the current state and the final state, but reduces the
overall transition risk.
This paper explains in further detail the need for such Transition States, and provides examples of
situations in which their deployment increases project resilience and reduces project risk. It
recommends that the use of Transition States is embraced, rather than just being accepted as an
adequate last resort.